(1) Field of the Invention
The present invention relates to a process for producing of aromatic polyimide film. More particularly, it relates to an aromatic polyimide film-making process having superior productivity.
(2) Description of the Prior Art
Aromatic polyimide film has been known for its excellent heat resistance and electrical as well as mechanical properties, and has been widely used as a base film for electric and electronic applications, elements for spaceships and many kinds of industrial uses.
Such films of aromatic polyimide are generally made by a dry process from an organic solvent solution of polyamic acid as a precursor thereof.
As to production processes for aromatic polyimide film, there are known as prior arts British Pat. Nos. 898,651, 903,271, 903,272, 1,155,541 and U.S. Pat. No. 3,179,614, wherein disclosed are methods consisting basically in casting a solution of polyamic acid over a support, treating it for ring closure by heating or by the use of a chemical reagent, then peeling the wet or gel film formed off the support and letting the remaining volatile content of the film vaporize by heating.
These known processes, however, invariably have very low solvent removal rates resulting in a low productivity, and further drawbacks such as thermal decomposition of solvent or the like when heated at a high temperature and increased costs because of the large amount of thermal energy needed etc., leading to inefficient productivity. Meanwhile, a wet process is disclosed in Japanese Patent Application Laid Open Publication No. 100460/1973 and in its Example 1 is described a process wherein a polyamic acid is fed to an extruder of 20 mm diameter preheated to 85.degree. C., and extruded as a film through its die under a back pressure of 55 kg/cm.sup.2.
The molten extruded film is fed into a water bath placed 25 cm under the die to be wound on a metal drum placed in the lower part of the bath. The water wetting the films surface is wiped off, and the film is fed to heat treating equipment. The temperature of the water bath maintained at 20.degree. C., and the residence time in water is 3.5 minutes. The film coming out of the water bath is self-supportable and its R.S. (residual solvent content) is 42%. The heat treating equipment is of the tenter type, being capable of treating for 1 minute at 200.degree. C. and 3 minuters at 350.degree. C. The polyimide film obtained has a ring closure ratio of 100%.
In the described process, however, the extruded film is immediately introduced into water, hence it is difficult to obtain a film of good surface smoothness.